In almost all cases today, coils for electrotechnical applications are wound during mass production. Because the wound enamel-insulated wires can exhibit defects, the occurrence of an interturn or even an interlayer fault cannot be overlooked. Interturn faults remaining undetected in the course of production can possibly cause long-term coil failure. Therefore, within the scope of quality assurance, it is necessary for the ready-wound coils to undergo a test for interturn and/or interlayer faults. Several measuring methods, following very diverse physical principles, can be used for this purpose.
In previous practice, customary measuring methods operate, for example, according to the principles of an a.c. voltage measuring bridge or an impulse voltage testing method. Measuring instruments produced according to these principles operate satisfactorily when the coils have only a small number of turns. However, when the coils have a large number of turns, the prior known instruments prove unreliable due to the parasitic coil capacitance or the minimal measuring effect caused by an interturn fault.
Due to the fact that increasingly higher demands are placed on coils, any interturn faults across a turn must be reliably detected. This is particularly true for coils having a very high number of turns. The measuring method must also be suitable for application in a flexible, large-scale coil production.
A method and apparatus for testing coils for interturn and/or interlayer faults is known from DE-PS 910 092 and DE-AS 26 44 253. In these references, the coil to be tested slides onto adjacent arms of U-shaped cores. Magnetic induction induced therein is used for measuring the short-circuited windings of any faulty coils. An advantage of these devices is that contact is not made with the coils that are tested. However in practice, contacting the coil connections is necessary in many cases during the course of quality control. For example, contacting is required for measuring copper wire resistance. Therefore, contact is generally included in the test of the coils anyway.
There is therefore needed other methods with which a winding test can be performed without any problems.